Depending on how an imposition was created in Prinergy, it may not be editable in Preps.
Any imposition created using an integrated Preps workflow is editable in Preps.
However, if you import any of the following types of imposition files into Prinergy, the imposition will not editable in Preps:

• PJTF from a stand-alone version of Preps
• MIS JDF with layout
• JDF and PJTF from other imposition applications (for example, Signa Station)